I have A Mod Honda that I have had for some time. It has A o-ring groove machined into the cylinder side of the head surface. Is this something an engine builder would of added? There Are A lot of “ID” numbers all over the motor ,but no Signature of who built it. The seal/o-ring is not listed on any websites in the parts breakdowns. If its an add on is it just A buma rubber o-ring ? Thanks

That’s part of what makes your engine “mod”, as opposed to stock. It’s there to keep the extra compression your engine has from blowing past the stock head gasket. I would use Viton for that o-ring, it gets hot where it is located.